adapted from: http://en.wikipedia.org/wiki/Painterly
Painterly is a translation of the German term malerisch, one of the opposed categories popularized by Swiss art historian Heinrich Wölfflin (1864 - 1945) in order to help focus, enrich and standardize the terms being used by art historians of his time to characterize works of art. The opposite character is linear, plastic or formal linear design. The term "painterly" serves to illuminate one aspect of painting when the viewer wants to begin to deepen his understanding of art. Painterliness in itself is insignificant and as such does not contribute to the quality of a work. Note that there is no absolute division between painterly and linear works. Painterliness in art is a matter of degree. Here is a hypothetical example rating painterliness among three well known artists: on a scale of 0 to 100 Willem DeKooning gets a 100, Edgar Degas a 60, and Edward Hopper a 30.
An oil painting is "painterly" when there are visible "brush strokes", the result of applying paint in a less than completely controlled manner, generally without closely following carefully drawn lines. Works characterized as either "painterly" or "linear" can be produced with any painting media, oils, acrylics, watercolors, gouache, etc. "Painterly" characterizes the work of Pierre Bonnard, Francis Bacon, Vincent van Gogh, Rembrandt or Renoir, John Singer Sargent and many others. In watercolor it might be represented by the early watercolors of Andrew Wyeth. "Linear", the term used to characterize the works of minimally painterly, some would say not painterly at all, artists such as Botticelli, Michelangelo or Ingres whose works depend on creating the illusion of a degree of three dimensionality by means of "modeling the form" through skillful drawing, shading, and a learned rather than impulsive use of color. Contour and pattern are more in the province of the linear artists while dynamism is the most common trait of painterly works.
The Impressionists and the Abstract Expressionists tended strongly to be "painterly" movements. Both Pop Art and Photo-realism, due to their dependence on photographic imagery, were characterized by an absence of apparent brushstrokes. The Pop artist Roy Lichtenstein made a painting which commented on Abstract Expressionist painterliness when he utilized images of brush strokes, rendered in a style reminiscent of a comic book, complete with Benday dots, in other words a flat looking painting spoofing the three dimensionality of Abstract Expressionism.
What Rembrandt is to light, Delacroix is to color. Colorists in rendering form, shadow, light and surface depend far more on subtle color relationships than do the artists who are less concerned with the subtleties of color and are more dependent on correct drawing and the accurate observation of both form and illumination. In neither case does it mean that the artists are slaves to "accuracy". "Painterly" art often makes use of the many visual effects produced by paint on canvas such as chromatic progression, warm and cool tones, complementary and contrasting colors, broken tones, broad brushstrokes, sketchiness, and impasto.
Finally, "painterly" refers to a certain use of paint in art. Additionally it happens that some forms of sculpture make use of apparently random surface effects which if not exactly resembling brushstrokes contain the traits of painterliness, (see Wood as a medium). The application of the term "painterly" outside of painting is justified and may help the viewer experience more deeply the significance of Auguste Rodin's surfaces or Richard Strauss's flow of chromatic harmonies.